The principal aim of these investigations is the study of the molecular structure and function of endogenous type C retroviral DNA. Using techniques of molecular cloning, nucleic acid hybridization, and nucleotide sequencing, nearly 40 murine leukemia virus-related proviral DNA segments have been cloned from a human gene library. These retroviral sequences can be grouped into two classes: approximately 25% have the properties of a full-length type C retroviral DNA and contain LTR, gag, pol, and env regions. Nucleotide sequencing has revealed the presence of several characteristic features of retroviral LTRs including inverted repeats, several regulatory signals affecting transcription, and the presence of an adjacent polypurine tract (near the 3 feet LTR) and a putative t-RNA binding site (5 feet LTR). Extensive nucleotide and amino acid sequence conservation of the gag and pol regions of human and murine proviral DNAs was observed. The human env region has a large open reading frame with six potential glycosylation sites but shares no homology with the comparable region of any known retrovirus. Normal and malignant human tissues have been screened for the presence of retrovirus-related RNAs using radiolabeled DNA probes derived from the LTR, gag, pol, and env segments of the cloned human endogenous retroviral DNAs. A tissue-specific pattern of expression has been observed employing RNAs prepared from human spleen, placenta, colon carcinoma, T-cell leukemia, retinoblastoma, and rhabdomyosarcoma cells, all of which contained human retroviral RNA as monitored by the dot-blot hybridization technique. Liver cells, B-cell lines, and several human carcinoma lines contained no detectable RNA. Northern blot analyses indicated that two different human placentas contained significant amounts of a 3.0 kb RNA species that anneals to LTR and env DNA probes. This reactive RNA, comigrated in a similar gel with the 21S spliced MuLV LTR-env RNA isolated from virus-producing cells. Additional RNA dot-blot and Northern analyses are in progress to establish the extent of endogenous retroviral expression in human cells.